pneumatic catapults

[traxxasracr1]

A few team members and I (who just graduated and are kinda bored) are making a robot for Stronghold out of leftover parts. I was curious as to what size pneumatic cylinders you guys used for your pneumatic catapults and how long your arm was/where the pistons mounted ect... Just so we can get an idea of where to start. Thanks for your input

[Wren Hensgen]

The last time I did this, we used some fairly large (2-7/8ths) bore cylinders to throw the 2014 Aerial Assist ball.

A couple things to keep in mind:

Assuming you're using a decently sized cylinder, it should be a priority for you to keep your hoses to a minimum, and use a solenoid with enough flow rate to handle the shot.

You can latch a cylinder in place (we used a gate latch for this), or brake it, so that the pressure has a moment to build behind the piston. We found that even a quarter second gave us a significantly harder shot. This also gives you an adjustment, varying the charge time by fractions of a second results in different powers and arcs.

It's better to have a solid mechanical stop for the arm of the catapult. Don't slam the cylinder against the end of the travel if you can avoid it.

You'll get a better exhaust rate, and thus, a better throw, if you simply don't retract the cylinder. Leave the top port empty in order to do this legally.


Good luck!

[Billfred]

Some other notes we've found:

- Multiple small cylinders will fill faster than one big one. (You can also plumb just one of these for retraction, if you don't want to do a spring return.)
- A 1" webbing strap makes a great hard stop for catapults.
- High cV is the order of the day. Automation Direct sells one that's aimed that way, but I don't know the part number offhand.

[GeeTwo]

In addition to the items above:

The volume of each cylinder (or more precisely, the volume filled by each solenoid valve in completing the stroke) is more important than the bore or stroke separately, though a short, wide cylinder will be connected nearer the axis of launcher rotation, and a long, thin one near (or past) the boulder.

IIRC when I did our calculations, somewhere around 8 or 10 cubic inches was optimal. This was using airflow through a standard 1/8" NPT port and a 12" ball movement during the launch; a high flow port or a longer throw would allow a larger cylinder, while a shorter ball movement or smaller port would require smaller volumes, and thus more solenoid valves.

For best results, include an air tank on the low pressure side of the regulator. If the air has to work its way through the regulator during the stroke, you won't get the advantage of high-flow or multiple solenoid valves.

[Tom Ore]

https://grabcad.com/library/2016-525-swartdogs-2

You can download our CAD from FRCDesigns to see what we did.

We had two tanks on the high pressure side and two on the low pressure side. We used two cylinders on the catapult. We varied the shot distance by changing the length of time the solenoids were on and by putting a delay start on one of the solenoids. This gave us a decent shot from a wide range of distances. It took a bit of calibrating to find the best settings for different distances. The competition bot didn't work quite as well as the practice bot because we never had time to really dial it in. We used the sames settings from the practice bot but even small changes in valves and the robot make a difference.

[Joey1939]

In 2014, my team used two large pneumatic pistons on a catapult to launch the ball. We quickly discovered that every little advantage you give the piston in extending quickly makes a large difference. A piston by itself couldn't launch the ball with enough force. First, pistons can extend faster than they can pull, so make sure the catapult is setup to shoot when the piston extends. Second, holding the piston back for a second lets it build up pressure and extend more quickly when released. We tried using a gate latch but had so much trouble getting the gate latch to release when we wanted (In retrospect we should have used a little piston). Then in the offseason we used an electromagnet, which worked amazingly.

[jspatz1]

Our Stronghold catapult used 1.06" diameter cylinders. The moment arm to the cylinder clevis was 4", and the arm length to the center of the ball was 27".

https://youtu.be/kKJDhcgQxkY?t=34

[ollien]

Originally Posted by Billfred

- Multiple small cylinders will fill faster than one big one.

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

[Andrew Schreiber]

Originally Posted by ollien

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

It's a flow problem. At what point in the system is the flow most restricted?

[ollien]

Originally Posted by Andrew Schreiber

It's a flow problem. At what point in the system is the flow most restricted?

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

[Andrew Schreiber]

Originally Posted by ollien

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

Try drawing out the whole system.

The max flow into a 1" bore cylinder is going to be exactly the same as into a .25" cylinder because the input is the constraining factor on both of them.

[adciv]

Originally Posted by ollien

I would argue that the flow would be more restricted when going to many different outlets, but I could be totally off base.

Primary constraint is the solenoid flow rate (CV rating). If you use multiple cylinders, you can use multiple solenoids. This gives you a higher effective CV rating.

[GeeTwo]

addciv got the key point, but was probably not clear enough for many. Let me be a bit more explicit:

Originally Posted by Billfred

Some other notes we've found:

- Multiple small cylinders will fill faster than one big one.

Originally Posted by ollien

What's the intuition for this? If we have four .25 L cylinders, shouldn't it take just as long to fill those as it does to fill a single 1 L cylinder?

Multiple small cylinders filled through the same solenoid valve will fill no faster than a single large one (and perhaps a bit more slowly, given the tubing required). I presume that Billfred is providing this in light of the recurrent rule:

Originally Posted by 2016 Game Manual

R90 The outputs from multiple valves must not be plumbed together.

That is, multiple smaller cylinders, EACH FILLED THROUGH A SEPARATE SOLENOID VALVE, will fill faster than one big one filled through (as required by the rules) one solenoid valve.

[ollien]

Originally Posted by GeeTwo

addciv got the key point, but was probably not clear enough for many. Let me be a bit more explicit:





Multiple small cylinders filled through the same solenoid valve will fill no faster than a single large one (and perhaps a bit more slowly, given the tubing required). I presume that Billfred is providing this in light of the recurrent rule:

That is, multiple smaller cylinders, EACH FILLED THROUGH A SEPARATE SOLENOID VALVE, will fill faster than one big one filled through (as required by the rules) one solenoid valve.

Maybe I'm misinterpreting the rule and the conclusions you're drawing from it, but no inspector has ever faulted us for having one solenoid for two pistons.

Why would filling two pistons from one solenoid, vs 2 pistons from 2 solenoids, be faster?

[Knufire]

Originally Posted by ollien

Maybe I'm misinterpreting the rule and the conclusions you're drawing from it, but no inspector has ever faulted us for having one solenoid for two pistons.

One solenoid for two cylinders is legal. Two solenoids for one cylinder is not legal.

Originally Posted by ollien

Why would filling two pistons from one solenoid, vs 2 pistons from 2 solenoids, be faster?

There used to be a rule that limited the Cv (flow coefficient) of solenoid valves on the robot, causing the solenoid to be a bottleneck in the flow path. That rule has since been removed. However, larger Cv solenoid valves are generally larger and/or heavier, so multiple cylinders on multiple solenoids may be a good alternative depending on your design constraints.